1. Field of the Invention
The present invention relates to an optical ophthalmic treatment apparatus, and more particularly to an optical ophthalmic treatment apparatus capable of applying a therapeutic laser beam onto an affected part of the eye, while the operator observes the eye to be operated through a stereo microscope.
2. Description of Related Art
A known optical ophthalmic treatment apparatus erodes a surface of a cornea with an excimer laser or the like to change the curvature of the surface of the cornea, whereby the refractive error of the cornea can be corrected. In such apparatus, to irradiate precisely a therapeutic light beam onto the eye, it is preferable that the optical axis of a therapeutic light beam, in a laser beam transmitting optical system within the apparatus, be coaxial with the optical axis of an objective lens in an observation optical system.
A method has been proposed arranging a dichroic mirror in the optical path of the therapeutic light beam.
For example, Japanese Laid-open Patent Application No. SHO 64(1989)-58255 proposes an apparatus providing a particular dichroic mirror which has a characteristic property of reflecting the laser beam from the mirror surface to erode a surface of the cornea, while reflecting the visible rays by upper and lower parts of its reverse portion. The part dichroic mirror is being arranged between the objective lens and the observation device in the well-known binocular observation optical system comprising the objective lens and the observation device, with the dichroic mirror being capable of reflecting infrared radiation by and transmitting the visible rays through the surface which is irradiated by a laser beam. The therapeutic light beam emitted from a laser beam oscillator is reflected by the dichroic mirror, inserted in the optical path of the binocular observation optical system toward the eye which is to be operated upon, and further condensed on the affected part of eye to be operated upon through the objective lens. In this apparatus, after reflected by the eye, the illuminating light for observation is introduced into the binocular observation optical system through the objective lens and the dichroic mirror.
In another proposed method, a total reflecting mirror is arranged in the optical axis of the objective lens to reflect the therapeutic light beam. For example, the apparatus comprising a stereo microscope for operation is shown in Japanese Laid-open Patent Application No. SHO 63(1988)-216566. In this apparatus, a pair of binocular observation optical systems are arranged to share an objective lens, and a total reflecting mirror is disposed on an optical path of the objective lens between the optical axis of each binocular observation optical system. Therefore, by using this apparatus, the front part of the eye, including the cornea as a portion to be operated upon, may be observed stereoscopically through the observing optical system, and the therapeutic light beam may be transmitted onto the affected part of the eye to be operated upon through the total reflecting mirror.
However, there are some problems in these known methods. In the former method using a dichroic mirror, higher apparatus cost results from the use of expensive dichroic mirrors. As shown in FIG. 4, in the case of arranging a dichroic mirror with a suitable inclination (normally 45.degree.) between an objective lens and a target on the patient's eye, astigmatism occurs. Therefore, an aberration compensating lens needs to be arranged in the observation optical system.
In the latter method using a small total reflecting mirror, an observation visual field is unobstructed by the total reflecting mirror because a small mirror may not enter each optical path of right and left eyes of an observing system. In the case of using a larger mirror, the edge of which appears at an inner side of each visual field of the right and left eyes, and a target on the patient's eye may thus not be observed clearly. Accordingly, because of the size of the total reflecting mirror to be employed, a part of the observation visual field is obstructed by the total reflecting mirror, resulting in a partial loss of the visual field.